A researcher says she has uncovered vital testimony from a U.S. officer who in 1943 was forced by the Nazis to watch as they exhumed thousands of Polish officers killed on Soviet leader Josef Stalin's orders.

At a news conference Wednesday in Warsaw, U.S. researcher Krystyna Piorkowska said she found the Paris-dated May 10, 1945, testimony of former American prisoner of war Lt. Col. John H. Van Vliet Jr. in the U.S. National Archives near Washington last November. The sworn deposition provides evidence of Soviet responsibility for the 1940 massacre of some 22,000 Polish officers in the Katyn forest and other places in what was then the Soviet Union. The Soviet Red Army had taken the Polish officers prisoner after invading eastern Poland in September 1939.

Another report that Van Vliet made a few days later in Washington is considered missing. This has fueled speculation that the U.S. government helped cover up Soviet responsibility for the massacre out of fear that saying the truth would anger Stalin, whom the Allies were counting on to help them defeat Germany and Japan in World War II.

For decades, Moscow had blamed the Nazis for the massacre, but then in 1990 admitted that Stalin had ordered it. The massacre further embittered Poland's relations with Moscow, which has refused to consider the massacre a genocide and has been reluctant to prosecute any living henchmen.

Van Vliet was a POW in Germany when he was taken to Katyn to see the evidence.

In the document that Piorkowska found, Van Vliet tells an interrogating U.S. officer that he saw the exhumation of some 3,500 corpses in tailor-made, little-worn Polish uniforms. They were all killed with a shot to the back of the head.

The area was under Soviet control then. Germany's Nazi regime discovered the graves in early 1943 after invading parts of the Soviet Union.